A Fourier transform spectrometer, commonly denoted by the initials FTS, is an instrument observing a scene with a relatively low spatial resolution—i.e. sampling point size—and a very fine spectral resolution, and is consequently typically coupled with a so-called “imaging” device, working in wideband, which device has a finer spatial resolution. The FTS is coupled with an imager in the aim of improving a spatial alignment or “registration” of the FTS instrument.
By way of example, in a weather observation satellite, the Fourier transform spectrometer makes it possible to observe the spectrum of light in the infrared/near-infrared range, and its spatial resolution can be of the order of 10 to 20 kilometers. The Fourier transform spectrometer is coupled with an imager of finer resolution, of the order of the kilometer, allowing a better physical analysis of the observed scene.
Known Fourier transform spectrometers comprise an analogue detector arranged in a pupil plane. More recent Fourier transform spectrometers—called imaging FTS—comprise, as a replacement for the analogue detector, a focal plane array detector placed in an image plane. Breakdown of the field of a sampling point—representing the final resolution of the instrument—into elementary pixels notably makes it possible to effect digital compensation for the effect of the field, or “auto-apodization”, or else to make use of an image mode, i.e. a mode making it possible to image the field of view of a sampling point with a fine spatial resolution. This image mode makes it possible to replace the imager. The physical pixels forming such a matrix are typically produced using technology commonly denoted by the initials CMOS (Complementary Metal Oxide Semiconductor) or CCD (Charge-Coupled Device) technology. In order to optimize the signal-to-noise ratio, the reading of the detector is carried out using spatial resolutions that vary according to the mode. Henceforth, the term “macropixel” denotes the “digital” resolution of the Fourier transform spectrometer, and the term “superpixel” denotes the “digital” resolution of the image mode. A superpixel can be formed by a physical pixel of the detector array, or else by a grouping of physical pixels, for example of 2×2 or 3×3 pixels, according to the spatial resolution required for the image mode. A macropixel defines the spatial resolution of the interferogram mode, and is in practice formed by a plurality of physical pixels of the detector array, for example arranged according to an alignment into one or more rows or into one or more columns of physical pixels of the detector array, into the direction of the interferometric fringes in order to maintain a good level of contrast inside each macropixel.
A drawback of such Fourier transform spectrometers is linked to the fact that they are particularly sensitive to non-uniformities of response in the field of view. Non-uniformities of response are mainly due to the detector—also called PRNU, the initials for Pixel Response Non-Uniformity—and to the optics.
According to a known technique, it is possible to carry out equalization for the interferometric data on average over the interferometric macropixel. This technique is described in detail below with reference to FIG. 1. This technique is particularly suitable for observed scenes of a homogenous nature.